The present invention relates to an IC tag of non-contact type for use in a personal identification, in commodity control, in physical distribution, etc.
The non-contact type IC tag mainly includes an IC chip and an antenna circuit, in the structures thereof. In manufacturing of such the IC tag, the resonance frequency between the IC chip and the antenna circuit is determined, by means of capacitance and inductance thereof. For adjusting the resonance frequency, adjustment must be made on the capacitance and the inductance, respectively. Ordinarily, the capacitance is a value, which is inherent or unique to the IC chip, and for the purpose of changing the value thereof, it is necessary to add a capacitance in the antenna circuit. Also, for the purpose of changing the inductance, it is necessary to increase and decrease the number of turns and/or change a circuit length of the antenna circuit.
Conventionally, the following measures are proposed for dealing with the problems mentioned above.
For example, Japanese Patent Laid-Open No. 2001-010264 proposes a measure of adding a capacitance in the antenna circuit. Also, Japanese Patent Laid-Open No. 2001-251115 proposes a method of making an adjustment by changing the length of the antenna circuit. Herein, the antenna circuit is so designed that the antenna circuit has the resonance frequency lower than a desired one, in advance, for the purpose of making an adjustment on the inductance thereof. And then, the adjustment is made upon the inductance through decreasing the number of turns and/or shortening the circuit length thereof.
Also, other means is proposed in Japanese Patent Laid-Open No. 2003-173426, for the purpose of obtaining a desired resonant frequency. Thus, in this patent document, there is proposed a method for adjusting characteristics; i.e., suppressing dispersion between the products on the desired characteristic due to a mutual inductance function by fitting two surfaces of antenna circuits upon the surfaces thereof, or enlarging a communication distance with an interrogator much more than that of the conventional one.
By the way, such the IC tag as mentioned above has the following drawbacks:
Thus, in the Japanese Patent Laid-Open No. 2001-010264, the capacitor and the resistor for use of adjustment are provided within the antenna circuit. Therefore, there are problems of increasing the number of parts and the manufacturing cost thereof, and obstructing reduction in size of the device.
The method shown in the Japanese Patent Laid-Open No. 2001-251115 has a problem that a yield rate is lowered, resulting in disturbing reduction in size of the device.
The resonance frequency (f) between the IC chip and the antenna circuit is determined by the capacitance (C) of the IC chip and the inductance (L) of the antenna circuit. Thus, the relationship between them can be expressed by the following equation (1):f=2π×1/(LC)1/2  (1)
As is apparent from this equation, making the inductance (L) large enables the resonance frequency to be lower. In adjustment of the inductance mentioned above, the inductance (L) must be large in advance, for the purpose of designing the antenna circuit to have a resonance frequency lower than the resonance frequency desired. For this purpose, the number of turns must be large. However, if the number of turns is increased, a yield rate of the circuit is lowered, resulting in disturbing reduction in size of the IC tag in a final form of the products.
In the Japanese Patent Laid-Open No. 2003-173426, the two surfaces of the antenna circuits are fitted onto each other, causing difficulties in the operations. Thus, such the structure of fitting the two surfaces of the antenna circuits requires the operations of forming circuit by a double-sided printing, and also attaching a single body of the antenna circuit, which is formed by a one-sided printing, onto each other. Therefore, this causes difficulties in the operations.